Emulator sdltrs TRS 80

sdltrs — Tandy TRS-80 Model I

The TRS-80 Model I is the second GenX-DOS bundle compiled from source to WASM (after Emulator-atari800). It runs sdltrs (Mark Grebe / Jens Guenther) — the SDL2 TRS-80 emulator, BSD-2-Clause, still actively maintained.

Why sdltrs

The shortlist:

1. lkesteloot/trs80 — a clean TypeScript/ESM emulator, easy to embed. It works, but renders the screen with a baked-in green-phosphor padding border (an overscan frame around the text) that fought the CRT bezel, and exposes no save-state. It was the starting point but got replaced.
2. No libretro TRS-80 core ships in the EmulatorJS-flavoured set, so the EJS route was out (unlike most recent additions — see Emulator-VICE-family for that path).
3. sdltrs — accurate, maintained, renders a plain 512×384 text area with no phosphor padding (so the canvas just fills the bezel cutout), and has a built-in save-state in C (trs_state_save / trs_state_load). Worth building.

Source: https://gitlab.com/jengun/sdltrs — the matching jengun GitHub repo 404s; GitLab is the live one.

The build

emscripten via emsdk (lives at ~/emsdk/), same toolchain as atari800. sdltrs is a flat src/*.c SDL2 program, so the recipe is a single emcc of all the sources — no autotools:

emcc $SRCS -O2 -DSDL2 -DROM_PATH='""' -sUSE_SDL=2 -sASYNCIFY \
  -sALLOW_MEMORY_GROWTH=1 -sINITIAL_MEMORY=32MB -sFORCE_FILESYSTEM=1 \
  -sMODULARIZE=1 -sEXPORT_NAME=createSDLTRS \
  -sEXPORTED_RUNTIME_METHODS="['FS','callMain','ccall','cwrap']" \
  -sEXPORTED_FUNCTIONS="['_main']" \
  --embed-file ../../level2.rom@/level2.rom -o ../out/sdltrs.js

$SRCS is every src/*.c including PasteManager.c — its X11 clipboard paths are #ifdef-guarded out under -DSDL2. Output: sdltrs.js (~190 KB) + sdltrs.wasm (~1.3 MB).

-sASYNCIFY and two yield patches

Like atari800, sdltrs runs a blocking continuous loop (z80_run(1), main() never returns), so -sASYNCIFY is non-optional. But ASYNCIFY only helps if the loop actually yields. sdltrs's throttle uses SDL_Delay, which busy-waits under emscripten and freezes the page. Two #ifdef __EMSCRIPTEN__ patches fix it:

• trs_interrupt.c, trs_timer_sync_with_host() — replace the SDL_Delay(...) throttle with emscripten_sleep(...), and call emscripten_sleep(0) even when no delay is due, so the loop yields once per timer tick no matter what.
• trs_sdl_interface.c, trs_get_event() — the paused-machine SDL_WaitEvent path busy-waits too; swap it for while (!SDL_PollEvent(&event)) emscripten_sleep(10);.

With those, emscripten_sleep unwinds the stack each tick, the browser paints, and the createSDLTRS factory promise resolves.

-sMODULARIZE load order

The build emits a createSDLTRS(moduleOverrides) factory. sdltrs.js must load before the inline boot code that calls it — a load-order slip gives "createSDLTRS is not defined", silently swallowed inside the boot's async IIFE.

The ROM

play.html boots with the Model I Level II BASIC ROM (12 288 bytes, md5 ca74822ebc2803c6635a55116ecd9539), embedded into the WASM at /level2.rom via --embed-file. It was extracted from the trs80-emulator npm package (dist/Model1Level2Rom.js) — note that model1Level2Rom is a latin1 binary string: Buffer.from(str, 'latin1'), not utf-8, or you get 17 210 corrupt bytes. © Tandy / Microsoft; bundled for emulator-only use (recorded in ATTRIBUTION).

The play.html contract

createSDLTRS({
  canvas,
  arguments: ['-m1', '-romfile1', '/level2.rom', '-noled', '-borderwidth', '0', '-scale', '2', '/game.cmd'],
  preRun: [m => m.FS.writeFile('/game.cmd', cmdBytes)],
});

-m1 = Model I, -noled -borderwidth 0 = a clean 512×384 (4:3) text area with no chrome (1024×768 at -scale 2). The .cmd is written to MEMFS and passed as a positional arg; sdltrs's trs_load_cmd runs it after reset and jumps straight into the program — no disk boot.

The canvas backing-store gotcha (cost an hour)

sdltrs uses the software video path (SDL_GetWindowSurface / SDL_UpdateWindowSurface = a raw putImageData into the canvas). Emscripten does not resize the canvas backing store on this path — it stays at the HTML default 300×150 — and width:100% CSS only sets the display size. So the 1024×768 surface lands in a 300×150 buffer and only the top-left corner draws. Fix: hard-set the backing store via HTML attributes — <canvas width="1024" height="768"> (= the scale-2 surface size); CSS then scales the display into the bezel cutout.

Why .cmd, not disk images

The lineup is 10 self-contained .cmd arcade games (Big Five Software / Adventure International). Disk boot was investigated and dropped: the RetroStore disk_0.* images aren't bootable system disks (the games ship as command.CMD to run on a bare machine), and a bare Model I with a non-bootable disk hangs on the black FDC boot-wait. The .cmd-as-positional-arg path sidesteps boot entirely.

Sourcing note: RetroStore's RPC (retrostore.org/rpc?m=pubapplist → downloadapp?appId=…) yields a ZIP with the self-contained command.CMD plus a disk image; we keep the .cmd.

CLEAR softkey

Most TRS-80 arcade games start with the CLEAR key, which has no obvious modern-keyboard equivalent. sdltrs maps CLEAR to PC Home (or Delete), so systems/_shared/genx-trs80-softkeys.js adds a top-left CLEAR button that dispatches a synthetic Home keydown/keyup to document/window/body/canvas — emscripten SDL2 doesn't check isTrusted, so the synthetic key reaches the emulator. Esc is the TRS-80 BREAK key.

Save / load state

sdltrs has a working save-state in C (trs_state_save.c), but it isn't wired to the page's save/load menus yet — so the TRS-80 bundle ships without them for now. The hooks are there for a future pass.

Bundle layout

systems/trs80/
├── play.html
├── sdltrs.js
├── sdltrs.wasm          ← Level II ROM embedded here
├── games.json
├── controls.html
└── games/               ← 10 .cmd programs

The build tree is ephemeral (/tmp); only a rebuild needs the recipe above — re-clone, re-apply the two patches, run the emcc line.

Related

• Emulators — index of all engines
• Emulator-atari800 — the other from-source ASYNCIFY emscripten build
• Emulator-XRoar-CoCo — the other Tandy machine